1. Field of the Invention
The present invention relates to thin-film coatings of semiconductor wafers and flat panel displays, and in particular to spin coating large surfaces with photoresist and similar high-viscosity chemicals.
2. Description of the Prior Art
In the fabrication of semiconductor devices, a wafer is coated with photoresist or a similar high-viscosity liquid chemical by spinning the wafer and applying an amount of the chemical to the center of the spinning wafer. (As used herein, the term "liquid" is not limited to fluids of low-viscosity and, in fact, can refer to highly viscous chemical substances.) Until recently, spin coating has been mostly confined to coating circular surfaces or small square mask plates. However, spin coating is now used to apply chemicals to thin film heads, multi-chip modules, and flat panel displays which are square or rectangular and are often very large. Spin coating such surfaces presents primarily two problems.
First, the corners of a square or rectangular surface are poorly covered. The chemical being applied to the surface is spun to the edge where excess is thrown from the surface by centrifugal force. Chemical leaving the center of an edge is struck by the corner of the spinning surface as the corner is further from the center of the surface than is the center of an edge. Such a collision disrupts the even flow of chemical from the center of the surface to the corners of the surface.
Secondly, evaporation of solvent from the chemical and the cooling of the chemical as a result of such evaporation increases the viscosity of the chemical. Therefore, as the chemical spreads toward the edges of the spinning surface, the thickness of the coating of chemical on the surface increases. Such thickening over time causes a "bowl" shaped coating in which the coating is thinner near the center of the surface and thicker near the edges of the surface.
One solution to the latter problem of the chemical drying too soon on the spinning surface is to apply the chemical to a surface spinning at a normal speed and then to ramp up, i.e., gradually increase, the speed of the spinning surface as the chemical spreads. Increasing the spin rate spreads the chemical more quickly and potentially before the chemical thickens substantially. However, increasing the spinning speed of the surface creates another problem.
The surface tension of the chemical on the spinning surface is what causes the chemical to spread evenly during spinning. However, as the spinning rate is increased, the centrifugal force overcomes the surface tension of the chemical. This is especially true on larger surfaces such as large flat panel displays. When the surface tension is overcome, the smooth circular shape of the spreading chemical bursts like a bubble and the chemical then streams linearly toward the edges of the spinning surface in multiple radial paths. These multiple radial paths form striations in the coating of the spinning surface.
The radial gaps in coating between the striations are filled in by applying an excess of the chemical to the center of the spinning surface until radial flow of the chemical from the center of the surface, as a result of the extreme centrifugal force, fills in the uncoated area. Thus, the surface is coated by saturating the surface with chemical, most of which is discarded as waste. Furthermore, the uniformity of the coating is poor as a result of the striations in and drying of the chemical coating.
Excessive use of photoresist in particular is a significant problem in the art. Photoresist accounts for approximately 5% of the cost of materials for semiconductor devices and generally costs as much as $1,000 per gallon. Thus, excessive waste of photoresist significantly affects the cost of manufacturing semiconductor devices. Additionally, disposal of photoresist waste presents a substantial environmental burden on communities in which semiconductor devices are manufactured and on surrounding ecological systems. The problem is exacerbated when coating larger surfaces.
One solution found in the prior art is illustrated by FIG. 1. An object 102 to be coated with a chemical on surface 104 was placed on a spinning chuck 106 which was spun by a motor 108. A chemical was deposited on the center of surface 104 through a tube 110. The drying of the chemical during spreading of the chemical by centrifugal force was slowed by containment of solvent vapors evaporating from the chemical. Containment of some of the solvent vapor was accomplished by placing a lid 112 over and in close proximity to surface 104 during spinning.
The apparatus of FIG. 1 did not stop drying of the chemical altogether, but rather slowed the drying by trapping solvent vapors evaporating from the chemical. The apparatus of FIG. 1 provided no way to control the rate of drying of the chemical. Furthermore, excess chemical was thrown by centrifugal force from surface 104 to the inner surface of lid 112. Therefore, it was necessary to periodically clean lid 112 and such cleaning increased substantially the introduction of particle contaminants into the system and reduced the feasibility of automatic, high-speed manufacturing.
FIG. 2 shows another apparatus used in the prior art to slow the drying of a chemical in the spin coating of large or non-circular surfaces. Chemical 202 was placed at the center of surface 104 and a plate 204 was brought into close proximity to surface 104 during spinning. The object of having plate 204 in close proximity to surface 104 was to contain the solvent vapors evaporating from surface 104. The apparatus of FIG. 2 was only partially effective as plate 204 was stationary, causing surface effects and vortices in the air directly adjacent to surface 104, thereby causing chemical 202 to dry unevenly and failing to adequately slow the drying of chemical 202.
What is needed is a method and apparatus which allows greater control of the evaporation of solvent from a chemical as the chemical is applied to and spreads over a spinning surface. What is further needed is a method and apparatus by which drying of a chemical during spin coating is substantially slowed without substantially increasing the risk of contamination of the coated surface.